The invention relates to a device for heat transfer, particularly in fluidized bed equipment, as well as a process for heat transfer in high-temperature processes for bulk material, particularly granulates or powders, in fluidized bed equipment.
In many high-temperature processes, heat must be supplied or discharged in order to control the reaction. This includes calcination of limestone to obtain quick lime and dehydration of gypsum to obtain plaster of Paris or anhydrite plaster, as examples of endothermic reactions, and roasting of such ores as molybdenite MoS2 and pyrite FeS2 to obtain their oxides, as examples of exothermic reactions.
The kinetics of the reaction require a controlled temperature progression in order to prevent incomplete or too fast reactions. Particularly in continuous exothermic reactions, there may be undesirable incipient fusion, sintering and by-products, or the equipment may even be destroyed.
The state of the art includes various approaches towards solving this complex problem.
In DE 25 11 944, a cylindrical fluidized bed reaction is proposed for combustion of coal granulate. The heat is discharged through heat exchanger ducts with air flowing through, arranged horizontally on top of one another. The ducts are curved into an involute shape, and connect the centrally located air feed duct to the concentric collector on the outer wall. By unwinding the involute in a circle, high and even packing is obtained, as well as low-stress deformation due to heat expansion. Due to the high packing density, this process cannot be used for fine-grain powders because bridging and clogging occurs between the heat-exchanger tubes. WO 97/07073 suggests using cylindrical vessels for endothermic dehydration of gypsum, with a concentric tube coil in the vessel wall that is heated that is heated by means of flue gases. A helical ribbon mixer to loosen up the gypsum is provided in the remaining space available in the inner chamber. This improves heat transfer, and improved economic efficiency compared to conventional processes should be achieved if the vessels are placed in a multi-stage arrangement. Nevertheless, the heat exchanger surface to be placed at the edge of the vessel is small and the improvement that can be achieved is marginal.
A conventional multi-stage hearth furnace [Ullmann] is still used to roast MoS2, where its single rack generates a temperature profile that can only be controlled rather inadequately by means of the combustion air and water injection. In order to improve the exchange between gas and solids and to convey the powder, a central agitator with arms is provided on each rack. Nevertheless, sintering and incomplete oxidation still occur.